the physics programmeof the alice experiment at the...
TRANSCRIPT
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The physics programme of the ALICE experiment at the LHC
Martignano (Lecce), 12-19 June 2006
Marco MontenoINFN Torino, Italy
Italo-Hellenic School of Physics 2006“The physics of LHC”
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Contents•• NucleusNucleus--nucleus collisions (and nucleus collisions (and pp--pp) at the LHC) at the LHC•• The ALICE experimentThe ALICE experiment•• Event characterizationEvent characterization•• Highlights on some physics topics *Highlights on some physics topics *
Soft physics I Soft physics I Particle abundances and spectraParticle abundances and spectra
Soft physics IISoft physics II strange baryons and elliptic flow strange baryons and elliptic flow
Heavy Heavy FlavoursFlavours and and quarkoniaquarkonia Jets Jets
•• ConclusionsConclusions
* Results of studies published on Physics Performance Report Vol.II, CERN/LHCC 2005-030
Lecture 2
Lecture 1
3
Nucleus-nucleus (and pp) collisionsat the LHC
4
One HI experiment with a pp program: ALICE
Two pp experiments with a HI program:
CMS and ATLAS
5
Collision
system
<L>/L<L>/L<L>/L<L>/L0000
(%)
107
Run time
(s/year)
LLLL0
(cm-2s-1)
√sNN
(TeV)
1034 *14.0pp
70-50 106 ** 7.710275.5PbPb
Running parameters:
σσσσgeom
(b)
0.07
Then, other collision systems:pA, lighter ions (Sn, Kr, Ar, O) and lower energies (pp @ 5.5 TeV)
*LLLLmaxmaxmaxmax(ALICE) = 1031 ** LLLLintintintint(ALICE) ~ 0.7 nb-1/year
The beams at the LHC machine
6
Why heavy ion collisions:the QCD phase diagram
Colliding two heavy nuclei at ultrarelativistic energies allows to create in the laboratory a bulk system with huge density, pressure and temperature (T over 100,000 times higher than in the core of the Sun) and to study its properties.
QCD predicts that under such
conditions a phase transition
from a system composed of
colorless hadrons to a
Quark Gluon Plasma (QGP)
should occur ( the QGP should
live for a very short time,
about 10-23s, or a few fm/c).
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Lattice QCD calculations
Indication of phase transition fromHadron gas (HG) to a QGPat Tc≅170 MeV εc≅1 GeV/fm3
• true phase transition or crossover?
• free QGP or intermediatephase of strongly interactingQGP (sQGP) ?
• Chiral symmetry restoration ?
QGP?
HG?
F.Karsch, E.Laermann and A.Peikert, Phys. Lett. B478 (2000) 447.
mu= md = ms
mu = md
mu = md ; ms >>>> mu,d
Stefan-Boltzmann
Limit of ideal gas
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New conditions created at the LHC
<0.2~0.5~1ττττ0 (fm/c)
4–101.5–4.0<1ττττQGP (fm/c)
2x1047x103103Vf(fm3)
15–404–52.5εεεε (GeV/fm3)
1500-3000850500dNch/dy
550020017s1/2(GeV)
LHCRHICSPSCentral collisions
Formation time τ0 3 times shorter than RHICLifetime of QGP τQGP factor 3 longer than RHICInitial energy density ε0 3-10 higher than RHIC
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10-6 10-4 10-2 100x
108
106
104
102
100
Q2
(GeV
2)
Probe initial partonic state in a new Bjorken-x range (10-3-10-5):
• nuclear shadowing,• high-density saturated gluon distribution.
Larger saturation scale (QS=0.2A1/6
√sδ= 2.7 GeV): particle production dominated by the saturation region.
The QGP at LHC might evolve from a Color Glass Condensate in the initial state of the collision.
A new kinematic regime
10
LHC
RHICSPS
(h++h-)/2
π0
17 GeV
200 GeV
5500 GeV=√sLO p+p y=0
At LHC hard processes contribute significantly to the total AA cross-section.
• Bulk properties are dominated byhard processes
• Very hard probes are abundantly produced.
Hard processes are extremely useful tools:
• Probe matter at very earlytimes.
• Hard processes can be calculated by pQCD
Heavy quarks and weakly interacting probes become accessible
... and more hard processes
LHC: σσσσhard/σσσσtotal = 98% (50% at RHIC)
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The ALICE experiment
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ALICE: the dedicated HI experiment
Solenoid magnet 0.5 T
Central tracking system:• ITS •TPC• TRD• TOF
MUON Spectrometer
Specialized detectors:• HMPID• PHOS
ZDC ~110 m on both sides of collision point
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Tracking: the major challenge for ALICE
Nch(-0.5<η<0.5)=8000Only a slice of ∆θ∆θ∆θ∆θ=2o is shown
TOF
RICH
PHOS
TRD
Event display for a central Pb-Pb collision as seen in ALICE. Tracking in the central barrel involves TOF, TRD, TPC, ITS.
ITS
TPC
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ALICE layout : tracking
Inner Tracking System (ITS):6 Si Layers (pixels, drift, strips)Vertex reconstruction, dE/dx-0.9<η<0.9
TPC
Tracking, dE/dx
-0.9<η<0.9
TRD(conceived for electron identification, but excellent tracking device)
15
Tracking efficiency
TPC
ITS+TPC+TOF+TRD
Good efficiency
down to very low pT !!!
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Combined momentum resolution
Transverse Momentum(GeV/c)
0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Tra
ns
ve
rse
mo
me
ntu
m r
eso
luti
on
(%
)
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
ITS + TPC
ITS + TPC +new TRD
Transverse Momentum(GeV/c)
10 20 30 40 50 60 70 80 90 100
Tra
ns
ve
rse m
om
en
tum
re
so
luti
on
(%
)
0
1
2
3
4
5
6
ITS + TPC
ITS + TPC +new TRD
at low momentum dominated by
- ionization-loss fluctuations- multiple scattering
at high momentum determined by
- point measurement precision- alignment & calibration (assumed ideal here)
resolution ~ 4 % at 100 GeV/cexcellent performance in hard region!
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Track impact parameter resolution
< 60 µµµµmfor pt > 1 GeV/c
It is crucial for the reconstruction of secondary vertices(identification of particles from their decay topology) Mainly provided by the 2 innermost layers (pixel cells) in the silicon central tracker (ITS = Inner Tracking System)
PIXEL CELL
z: 425 µm
rφ: 50 µm
Two layers:
r = 4 cmr = 7 cm
×9.8 M
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ALICE: an ideal soft particle tracker
Magnetic field (T)
PT cutoff
(GeV/c)
Material thickness:
X/X0 (%)
ALICE 0.5 0.15 7
ATLAS 2.0 0.5 30
CMS 4.0 0.75 20
LHCb 4Tm 0.1* 3.2
o ALICE is sensitive down to very low PT
o Moreover ALICE has remarkable capabilities of particle
identification
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Alice uses ~ all known techniques!
0 1 2 3 4 5 p (GeV/c)
1 10 100 p (GeV/c)
TRD e /πPHOS γ /π0
TPC + ITS(dE/dx)
π/K
π/K
π/K
K/p
K/p
K/p
e /π
e /π
HMPID (RICH)
TOF
ALICE Particle Identification
EMCAL
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PHOton Spectrometer: PHOS
• High granularity detector:
– 17920 lead-tungstate crystals (PbWO4), 5
modules (56×64)
– crystal size: 22 × 22 × 180 mm3
– depth in radiation length: 20
• Distance to IP: 4.4 m
• Acceptance:
– pseudo-rapidity [-0.12,0.12]
– azimuthal angle 100o
• Energy resolution ~ 3% / √ E
• Dynamic range from ~ 100 MeV to ~ 100 GeV• Timing resolution of ~ 1.5 ns / √ E• Trigger capability at first level
• Charged Particle Veto, CPV
– multi-wire particle gas chamber
High resolution spectrometer
CPV
CrystalsEMC
21
Dimuon Spectrometer
• Study the production of the J/Ψ, Ψ', ϒ, ϒ' and ϒ'’ decaying
in 2 muons, 2.5 <η < 4
• Resolution of 70 MeV at the J/Ψ and 100 MeV at the Υ
Complex absorber/small angle shield system to minimize background(90 cm from vertex)
22
Proposed ALICE EMCAL
• EM Sampling Calorimeter (STAR Design)
• Pb-scintillator linear response
– -0.7 < ηηηη < 0.7
– 60606060°°°° < ΦΦΦΦ < 180180180180°°°°
• Energy resolution ~15%/√E
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With its system of detectors ALICE will meet the challenge to measure event-by-event the flavour content and the phase-space distribution of highly populated events produced by heavy ion collisions:
• Most (2π * 1.8 units of η) of the hadrons (dE/dx + TOF), leptons (dE/dx, transition radiation, magnetic analysis) and photons (high resolution EM calorimetry).
• Track and identify from very low pt (~ 100 MeV/c; soft processes) up to very high pt (>100 GeV/c; hard processes).
• Identify short lived particles (hyperons, D/B meson) through secondary vertex detection.
• Identify jets.
Summary of the ALICE features
24
0 1 2 10 100
pt (GeV/c)
Bulk propertiesHard processes
modified by the medium
ALICE
CMS&ATLAS
PID
T=ΛΛΛΛQCD Qs
pt coverage: ALICE vs CMS and ATLAS
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ALICE Physics Goals
• Event characterization in the new energy domain (for PbPb but also for pp)– multiplicity, η distributions, centrality
• Bulk properties of the hot and dense medium, dynamics of hadronization– chemical composition, hadron ratios and spectra, dilepton continuum,
direct photons• Expansion dynamics, space-time structure
– radial and anisotropic flow, momentum (HBT) correlations • Deconfinement:
– charmonium and bottomonium spectroscopy• Energy loss of partons in quark gluon plasma:
– jet quenching, high pt spectra– open charm and open beauty
• Chiral symmetry restoration: – neutral to charged ratios– resonance decays
• Fluctuation phenomena, critical behavior:– event-by-event particle composition and spectra
Not covered by these lectures
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Event characterization in ALICE
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ZDC and centrality determination
rapidity
y=0
b
participants
spectators
spectators
We measure in the ZDC for each event the
energy carried by spectators (individual
neutrons, protons and fragments)
If we can determine from it the number of
projectile spectators Nspect, then :
Npart =A – Nspect centrality estimate
Then, with a Glauber superposition model, Npart can be converted to b,
or to % of inelastic cross section.
ZP
ZN
~100 m from interaction point
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ZDC + ZEM for centrality determination
• EZDC correlated with number of spectators
BUT two branches in the correlation
• Break-up of correlation due to production of
fragments (mainly in peripheral collisions)
The ZEM has a signal with relatively
low resolution, but whose amplitude
increases monotonically with centrality.
% of σinelastic
AMBIGUITY
Which Nspect
for a given
EZDC ??
• An additional calorimeter (ZEM =
Zero Electromagnetic Calorimeter) at
7m from interaction point, is used to
solve the ambiguity.
The correlation between signals
measured by ZDC and ZEM allows
to determine centrality and define
some centrality bins.
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Centrality classes in Npart
Event by event determination of the centrality
(EZDC , EZEM ) Nspec Npart b
•Typical resolutions:
•σσσσNpart/Npart ~ 5 % for very central collisions
•σσσσNpart/Npart ~ 25 % for semi-peripheral collisions
(b ~ 8fm)
No significant bias in the reconstr.
RMS < separation between classes
10 centrality bins can be safely defined
bgen (fm)
bre
c(f
m)
Multiplicity measurements in ALICE
• Two detectors:
• SPD (Silicon Pixel Detector, 2 cylindrical layers) in the central region;
• FMD (Forward Multiplicity Detector, 5 rings of silicon strips) at
forward rapidities.
SPDSPDWe have chosen two relatively simple and fast algorithms:
1) cluster counting on each pixel layer (|η|<2 first layer, |η|<1.4 second layer);
2) counting of tracklets (association of clusters on the two layers, aligned with the
estimated vertex position) (|η|<1.4)
FMDFMD (-3.4<η<-1.7 and 1.7<η<5.1)
Reconstruction of multiplicity based on empty pad counting.
2 4 6-2-4-6η
0
Multiplicity
measurement
Multiplicity
measurement
Momentum
& Multiplicity
measurement
Arb
itrary
units
2
4
6
8
10
12
TPC
ITS Pixel
FMD FMD
The charged particle multiplicity is measured over 8.8 rapidity units, whereas the momentum is measured in the TPC (and in the Inner Tracking System) over 1.8 rapidity units with optimal resolution.
R. Caliandro, R.Fini and T.Virgili, Measurement of multiplicity and dN/dη using
the ALICE Silicon Pixel Detector, ALICE Internal Note ALICE-INT-2002-043
Clusters lay.1
Tracklets
Accuracy on the
reconstructed
Multiplicity:
Clusters on layer 1
and tracklets
BOTH slightly
underestimate
multiplicity
Difference <10%
Reconstructed
multiplicity distribution
with tracklets (HIJING
events)
Multiplicity reconstruction in PbPb with the SPD
•We can have in ALICE a coverage up to ~ 10 ηηηη-units
•Accuracy ~ 7% in 0.1 ηηηη bins
1
Central Pb-Pb
HIJING event
Reconstruction of dN/dη distribution in Pb-Pb with FMD+SPD
Kharzeev-Nardi model [Phys. Lett. B 507 (2001) 79]
Multiplicity
measured with SPD
10 centrality bins
in Npart estimated
event by event from
the measured EZDC
Fit with KN model
PHOBOS PRC 2004, nucl-ex/0405027
Study of multiplicity versus centrality (à la PHOBOS)
34
dNch/dη and energy density at RHIC
When assuming a formation time τ0=1fm/c the conditions reached at RHIC are:
> 5.0 GeV/fm3 for AuAu @ 200 GeV
> 4.4 GeV/fm3 for AuAu @ 130 GeV
> 3.7 GeV/fm3 for AuAu @ 62.4 GeV
Whereas the energy density reached at the CERN SPS was:
2.7 GeV/fm3 for PbPb @ 17.2 GeV
To be compared with the energy density predicted by Lattice QCD
εεεεC = 0.6 GeV/fm3
for the QGP formation. P. Staszel (BRAHMS), QM2005
Measurements of charged particle density at midrapidity can be used
to estimate the energy density created in a heavy ion collisions using
the Bjorken formula: εεεεBJ = 3/2 ××××(<Et>/ ππππR2τ0) dNch/dηηηη
Saturation model
Armesto, Salgado, Wiedemann hep-ph/0407018
Models prior to RHIC dN/dη ~ 1100
dN/dη ~ 1800
Log extrapolation
ALICE Detectors
are planned
to deal with:
dN/dηηηη > 5000
Energy dependence of multiplicity: extrapolation to LHC
36
Highlights on physics topics
Soft Physics I
Particle abundances Particle spectra and their ratios
Particle yields and chemical freezout at RHIC
Tch ≈ 165 ± 10 MeV ~ Tcrit (LGT)µµµµB ≈ 25 MeV
STAR white paper
Nucl Phys A757 (05) 102
short
liv
ed
resonances
Particle yields are established at the chemical freezout, and provideinformation on this relatively early stage (before hadronic expansion,cooling and “thermal freeze-out”)
Statistical thermal models (employing hadronic degrees of freedom in a grand-canonical ensemble) are able to reproduce the particle ratios using only two parameters: T (chemical freezout temperature)
µµµµB (baryo-chemical potential)
From the fit of the STAR data:
The ability of the stat.modelto fit the particle yields leads us to conclude that the experimental data at RHIC (and SPS) show a high degree ofchemical equilibration
At RHIC:
T = 177 MeV
T ~ Tcritical (QCD)
Unified statistical
picture from SISto RHIC.What about LHC ?
QCD phase diagram
39
Kinetic freezout at RHIC
STAR: Nucl. Phys. Rev. A757 (2005) 102
Thermal fit (using an inverse slope parameter 1/T ) of the particlespectra provides information on the dynamics of the expansion and the T of the system at the kinetic decoupling (thermal freezout), when particle ceaseinteracting. Models based on hydrodynamics can be used to estimate Tfo (freezout) and the mean transverse (radial) flow velocity <ββββT> simultaneously, and to studytheir behaviour as a function of centrality and particle type.
√√√√s=200 GeV
Tkin freezout~ Tchemical freezout (from statist. model analysis)
(ππππ, K, p) acquire strongercollective flow during the coolingphase between Tkin and Tch
Multi-strange hadrons (in the5% most central collisions) :
no overlapping χ2 contour with(π, K, p) at the same centrality
40
Ratios of hadron spectra: RCP
baryons mesons
RC
P
Binary scaled ratio of hadron yields in a central over a peripheral bin(the spectra are normalized to Ncoll, the number of binary collisionsin each centrality bin)
Different suppression pattern for mesons and baryons at intermediate pT
For baryons the suppressionis smaller. This supports the constituent quark coalescence(recombination) model
√√√√s=200 GeV
STAR: Nucl. Phys. Rev. A757 (2005) 102
ηηddpNd
ddpNd
N
NpR
T
peripheral
T
central
central
coll
peripheral
collTCP
/
/)(
2
2
><><
=
41
• The in vacuo fragmentation of a high momentum quark to produce hadrons competes with the in medium recombinationof lower momentum quarks to produce hadrons
• Example: creation of a 6 GeV/c π or p
– Fragmentation: Dq→h(z)
•produces a 6 GeV/c πfrom a 10 GeV/c quark
– Recombination:
•produces a 6 GeV/c πfrom two 3 GeV/c quarks•produces a 6 GeV/c protonfrom three 2 GeV/c quarks
...requires the assumption of a thermalized
parton phase... (which) may be
appropriately called a quark-gluon plasma
Fries, et al, PRC 68, 044902 (2003)
Coalescence: possible mechanism at intermediate pT
Recombination yields more baryonsat a given pT since the three combining quarkshave a larger yield at pT/3 compared to the two q at pT/2
B.Hyppolite
42
Baryon excess at intermediate pT
1) Low pT limit can be define
by the validity range of Hydro.
2) At high pT limit, the baryon
and meson should merge again.
At RHIC, a high pT limit at 6
GeV/c is found.
ΛΛΛΛ/K0s
Intermediate pT range:
In order to describe the magnitude of the baryon/meson ratio and the location of the maximum the contribution of both coalescenceand the mass-dependent radial flow is needed. So the fragmentationdominance for baryons starts at higher pTThis effect increases with centrality .
43
What to expect at LHC energies
Fries and Müller, EJP C34, S279 (2004)
Calculation in the scenario
recombination + pQCD fragmentation
must imply an assumption on transverse
radial flow extrapolation
Amplitude of the ratio at the LHC is the
same as at RHIC but
the limit of the intermediate pT region
is pushed to higher pT
Tfreezout =175 MeV, ββββT = 0.75
But still within the
ALICE pT reach !!
44
Ratios of hadron spectra: RAA
Nuclear Modification Factor RAA:AA = Nucleus-NucleusNN = Nucleon-Nucleon
ηηddpNdT
ddpNdpR
T
NN
AA
T
AA
TAA/
/)(
2
2
=
Nuclear overlap integral:
# binary NN collisions / inelastic NN cross section
NN cross section
AA cross section
AA
(pQCD)
Parton energy loss
→→→→ R < 1 at large Pt
45
Centrality Dependence of RAA
- Dramatically different and opposite centrality evolution of Au+Au experiment from d+Au control.
- High pT hadron suppression is clearly a final state effect (partonic energy loss in a dense medium) and not an initial state effect (gluon saturation).
- R dAu > 1 at intermediate pT shows a Cronin enhancement (multiple scattering)
Au + Au Experiment d + Au Control Experiment
Preliminary DataFinal Data
46
pT range (PID or stat. limits) for 1 year: 107 central Pb-Pb (or 109 min. bias pp)
ππππ, K, p: 0.1- 0.15 50 GeV (by using relativ.rise of dE/dx in the TPC) Weak or strong decaying particles: up to 10-15 GeV
Mid-rapidity
Pb-Pb
Particle identification range with ALICE (in one year)
47
BEGIN OF THE SECOND LECTURE
48
Contents
•• NucleusNucleus--nucleus collisions (and nucleus collisions (and pp--pp) at the LHC) at the LHC•• The ALICE experimentThe ALICE experiment•• Event characterizationEvent characterization•• Highlights on some physics topics *Highlights on some physics topics *
Soft physics I Soft physics I Particle abundances and spectraParticle abundances and spectra
Soft physics IISoft physics II strange baryons and elliptic flow strange baryons and elliptic flow
Heavy Heavy FlavoursFlavours and and quarkoniaquarkonia Jets Jets
•• ConclusionsConclusions
* Results of studies published on Physics Performance Report Vol.II, CERN/LHCC 2005-030
Lecture 2
Lecture 1
49
Highlights on physics topics
Soft Physics II
Strange baryons Elliptic flow
An enhancement in the production of rare strange hadrons was among the first signatures proposed for QGP observation in relativistic heavy ion collisions J.Rafelski and B. Müller, Phys. Rev. Lett 48 (1982) 1066; Phys. Rev. Lett.
56 (1986) 2334.
If deconfinement has occurred, the reactions leading to strangeness production are partonic: they have lower thresholds and higher cross sections, especially if the strange quark mass reduces owing to the associated partial restoration of the chiral simmetry.
Strangeness enhancement of single-strange particle observed in nucleus-nucleus collisions already at rather low energies
Multi-strange baryon enhancement observed at SPS (and then at RHIC)
Strangeness enhancement
NA57
Enhancement = yield per participant nucleon in PbPb relative to pBe
An enhancement, increasing with the centrality of the PbPb collision, has been observed in NA57 for various strange baryons at midrapidity
Ω enhanced by a factor 20
Strangeness enhancement in PbPbat the CERN SPS
STAR vs NA57
(QM2005)
Enhancements are the same or even bigger at RHIC than at SPS
Strangeness enhancement in AuAu at RHIC
53
K0s -Λ
b is the
DCA to the
prim. vertex
Daughter
DCA
Angle between
V0 momentum and
the VP direction
Secondary vertex reconstruction in ALICE Hyperon decays with V0 topologies
Reconstruction of the primary vertex P with hitsassociation in the 2 innermost silicon-pixel layers
Selection of secondary tracks (in TPC) via cutson impact parameter b with respect to the primary vertex (tracks rejected for b too small)
Combination of each secondary track with all the others with opposite charge. Numerical calculation of DCA (distance of closest approach).A pair is rejected when DCA is too large.The half-point at the minimum DCA is the localization of the vertex V of the V0 candidate.
Only vertices V inside a fiducial volume are kept.Its maximum size depends on a compromise between- need for a low background (small radius)- need for high reconstruction rate (bigger radius)
Checks whether the momentum of the candidateV0 points well back to the primary vertex P. A cut on the cosine of the angle between V0 momentum and the VP direction is applied.
bP
V
Finding procedure based ona topological identification(L. Gaudichet)
%)64B.R.(: ≈→Λ −πpEx
54
Example: Λ reconstruction in ALICE
ΛΛΛΛ
ΛΛΛΛ
Pb-Pb central
300 HIJING
events
Stat. limit11-12 GeV
Reconst. Rates:
ΛΛΛΛ: 13 /event
1 year Pb-Pb statistics
55
Ξ - Ω
Λ
V0 cuts,
selecting Λ
Bachelor
DCA to
primary
vertex
DCA V0-
Bachelor
Angle between
cascade
momentum and
primary vertex
Secondary vertex reconstruction in ALICE Hyperon decays with cascade topologies
The search of the V0 candidate (from the decay of the cascade particle) allows that it has a large impact parameter with respect to the primary vertex
The V0 candidates within the mass window have to be combined with all possible “bachelors”. The association is accepted if the DCA is small enough.
Checks whether the momentum of the Candidate cascade points well back to the primary vertex P. A cut on the cosine of the angle between cascade momentum and the VP direction is applied.
%)64B.R.( ≈→Λ −πp
%)100B.R.(: ≈Λ→Ξ −πEx
56
Multi-strange hyperon reconstruction in ALICE
ΩΩΩΩΞΞΞΞ
5000 Pb-Pb events300 Pb-Pb events
ΞΞΞΞ ΩΩΩΩ
1 year Pb-Pb statistics
Stat. limit8-10 GeV
Reconst. Rates:
ΞΞΞΞ: 0.1 /eventΩΩΩΩ: 0.01 /event
57
Anisotropic transverse flowAnisotropic transverse flow• In non-central nucleus-nucleus collisions, at t=0: geometrical anisotropy (almond shape) momentum distribution isotropic
• Interactions among constituents generate a pressure gradient which transforms the initial spatial anisotropy into a momentum-space anisotropy (that is observed as an azimuthal anisotropy of the outgoing particles)
x
z
y
y
x
• The mechanism is self quenching The driving force dominate at early times Sensitive to Equation Of State at early
times (t<5 fm/c)
py
px
Reaction
plane xz
58
Elliptic flow coefficient vElliptic flow coefficient v22• Experimentally the anisotropic transverse flow (also called elliptic flow) is measured by taking the second Fourier component of the particle azimuthal distributions relative to the reaction plane:
View along beamline
( )( )( )....2cos2)cos(212
210 +Ψ−+Ψ−+= RPRP vv
X
d
dXϕϕ
πϕ
REACTION PLANEREACTION PLANE = plane
defined by beam direction and
impact parameter
x
y
ΨRP
( )( )RPv Ψ−= ϕ2cos2
Elliptic flow coefficient
59
vv22 versus centrality at RHICversus centrality at RHIC
• Observed elliptic flow depends on: Initial eccentricity (decreasing with increasing centrality)
Amount of rescatterings (increasing with increasing centrality)
• Measured v2 well described by hydrodynamic modelfrom mid-central to central collisions Incomplete thermalizationfor peripheral collisions
Hint for rapid and complete thermalization for mid-central and central collisions
• Flow larger than expected from hadronic cascade models (like RQMD): Evidence for a strongly interacting (partonic) phase
Hydrodynamic limit
STAR
PHOBOS
Hydrodynamic limit
STAR
PHOBOS
RQMD
STAR: Phys. Rev. Lett. 86 (2001) 402.
PHOBOS: Phys. Rev. Lett. 89 (2002) 222301.
√√√√s=130 GeV
60
vv22 versus versus ppTT at RHICat RHIC
At low transverse momenta the elliptic flow is well describedby hydrodynamical models incorporating a softening of the Equationof State due to quark and gluon degrees of freedom
Deviations at high pT where: Hydrodynamics not applicable because high pT partons have not undergone
sufficient re-scatterings to come to thermal equilibrium Parton energy loss in the opaque medium is a source of anisotropy
STAR: Phys.Rev.Lett. 90 (2003) 032301.
61
HadronHadron vv2 2 and quark recombinationand quark recombination
Complicated pattern of v2 at higher pT:• lighter mesons deviate from hydrodynamics at pT ~ 1 GeV/c
• heavier baryons deviate at significantly higher pT
• baryons have higher v2values than mesons at the highest pTmeasured
When v2 is plotted per quark (v2/n) (n=3 for baryons, and 2 formesons) the values of v2/n scales with pT/n .
o Recombination model works nicely also for v2 o Evidence for early collective flow at the quark level !!!
62
Elliptic flow at the LHCElliptic flow at the LHC
• Multiplicity larger than at RHIC by a factor 1.5-2
• v2/ε expected larger than at RHIC Few predictions:
Teaney, Shuryak, Phys.Rev.Lett. 83 (1999) 4951.
Kolb, Sollfrank, Heinz, Phys.Rev. C62 (2000)
054909.
Bhalerao et al., Phys.Lett. B627 (2005) 49
• Large flow values 5-10% are expected
Easier measurement (feasible on day 1 !!)BUT: larger non-flow contribution from jetscould obscure the flow signal
Important to compare different methods
At LHC, contribution from hydrodin. flow in the QGP phase (down to chemical freezout)much larger than at RHIC
63
Experimental methods to estimate Experimental methods to estimate vvnn
Event plane method (Poskanzer and Voloshin, Phys. Rev. C58 (1998) 1671.)
Calculate an estimator of the reaction plane (EVENT PLANE) from the anisotropy of particle
azimuthal distributions
Correlate azimuth of each particle with the event plane calculated with all the other particles
WEAK POINT: assumes that the only azimuthal correlation between particles is due to their
correlation to the reaction plane (i.e. to flow)
BUT other sources of correlation (NON-FLOW) are in due to momentum conservation,
resonance decays, jets + detector granularity SYSTEMATIC UNCERTAINTY
Two particle correlations (S. Wang et al, Phys. Rev. C44 (1991) 1091.)
No need for event plane determination
Calculate two-particle correlations for all possible pairs of particles
WEAK POINT: same bias from non-flow correlations as in event-plane method
“Cumulants” method (Borghini et al, Phys Rev C 63 (2001) 054906.)
Extract vn from multi-particle azimuthal correlations
Based on the fact that flow correlates ALL particles in the event while non-flow effects
typically induce FEW-particle correlations
DRAWBACK: larger statistical error and more sensitivity to fluctuation effects
Lee-Yang zeroes method (Bhalerao et al, Nucl. Phys. A727 (2003) 373.)
Extension of cumulants method to infinite order
F. Prino
64
Event plane methodEvent plane method
• Event plane resolution depends on: Amount of anisotropy (v2) Number of used tracks
Reconstructd v2 vs. pT for TPC tracks • 100 Pb-Pb events • 2000 tracks per event
Performance of event plane methodvn = <cos[n(φφφφ-φφφφR)]> / ev.plane resolution
In ALICE various independent estimatesof reaction plane and vn from trackanisotropy in different regions of phase space
Measurements with: TPC/ITS, SPD (2 layers of Silicon Pixel Detectors) and forward detectors (FMD, PMD)
TPC
65
Highlights on physics topics
Heavy flavours and quarkonia
66
Heavy Flavour physics in ALICE: motivations
• Energy loss of Heavy Quarks (HQ) in hot and high density medium formed in AA central collisions.
• Brownian motion and coalescence of low pT HQ in the quark gluon plasma (QGP).
• Dissociation (and regeneration) of quarkonia in hot QGP.
• Heavy flavour physics in pp collisions: small x physics, pQCD, HQ fragmentation functions, gluon shadowing, quarkonia production mechanism.
67
c and b production in pp at the LHC
Important test of pQCD in a new energy domain (14 TeV)
CDF, PRL91 (2003) 241804FONLL: Cacciari, Nason
Cacciari, Frixione, Mangano, Nasonand Ridolfi, JHEP0407 (2004) 033
σ
Cacciari, Nason, Vogt, PRL95 (2005) 122001
Q → e±+X
At Tevatron (1.96 TeV) B production is well described by a FONLL
calculation of
But… charm production is still underpredicted at Tevatron (1.96TeV)
and at RHIC (200 GeV)
68
increasing √s
Heavy quarks as a probe of small-x gluons
Probe unexplored small-x region with HQs at low pt and/or forward y
down to x~10-4 with charm already at y=0
Eskola, Kolhinen, Vogt, PLB582 (2004) 157Gotsmann, Levin, Maor, Naftali, hep-ph/0504040
Cartoon
char
mbea
uty
Window on the rich phenomenology of high-density PDFs
gluon saturation / recombination effects
non-linear terms in PDFs evolution
Possible effect: enhancement of charm production at low pt w.r.t. to standard
DGLAP-based predictions
σc(DGLAP+non-linear)
σc(DGLAP)
A. Dainese
69
Hard primary production in parton processes (pQCD)Binary scaling for hard process yield:
Baseline predictions for charm / beauty:
NLO (MNR code) in pp + binary scaling (shadowing included for PDFs in the Pb)
Secondary (thermal) c-cbar production in the QGPmc (≈1.2 GeV) only 10%-50% higher than predicted temperature of QGP at the LHC (500-800 MeV)
Thermal yield expected much smaller than hard primary production
TppcollTAA pNNpN d/dd/d ×=
c and b production in A-A collisions at the LHC
0.16 / 0.0070.8 / 0.03115 / 4.6
--0.84 / 0.900.65 / 0.80
11.2 / 0.57.2 / 0.34.3 / 0.2
pp
14 TeV
p-Pb (min. bias)
8.8 TeV
Pb-Pb (0-5% centr.)
5.5 TeV
system :
√sNN :
[mb] QQ
NNσQQ
totN
98EKS
shadowingC
!!2010 cc
RHIC
cc
LHC σσ ×−≈
shadowing
antishadowing
SPSSPS
RHICRHIC
LHCLHC
Initial state effectsPDFs in nucleus different from PDFs in nucleon
Anti-shadowing and shadowing
kT broadening (Cronin effect)
Parton saturation (Color Glass Condensate)
Final state effects (due to the medium)Energy loss
Mainly by gluon radiation
In medium hadronization
Recombination vs. fragmentation
Present also in pA (dA) collisions
Concentrated at lower pT
Only in AA collisions
Dominant at higher pT
A
A
c
c
D
Du
Violation of binary scaling in A-A collisions
71
Pb-Pb collisions: parton energy loss
Partons travel ~ 4 fm in the high colour-density medium
Energy loss (gluon radiation+ collisional?)
2 ˆ LqCE Rsα∝∆
Casimir coupling factor:4/3 for quarks
3 for gluons
Medium transport coefficient
∝ gluon density
Baier, Dokshitzer, Mueller, Peigne‘, Schiff, (BDMPS), NPB483 (1997) 291
(BDMPS)
Probe the medium
72
Lower E loss for heavy quarks?
In vacuum, gluon radiation suppressed at θ < mQ/EQ
→→→→ “dead cone” effect
Dead cone implies lower energy loss (Dokshitzer-Kharzeev, 2001)
Detailed calculation confirms this qualitative feature,
although effect is small and uncertainties significant (Armesto-
Salgado-Wiedemann, 2003)
Exploit abundant massive probes at LHC & study the effect
by measuring the nuclear modification factor for D and B
Q
Dokshitzer, Kharzeev, PLB519 (2001) 199Armesto, Salgado, Wiedemann, PRD69 (2004) 114003
t
BD
pp
t
BD
AA
coll
t
BD
AAdpdN
dpdN
NpR
/
/1)(
,
,, ×=
222 ])/([
1
QQ Em+∝
θ
Gluonstrahlung probability
73
Heavy-flavours in ALICE
• ALICE can study several channels:– hadronic (|η|<0.9)– electronic (|η|<0.9) – muonic (2.5 < η < 4)
• ALICE coverage: – low-pT region (down to
pt ~ 0 for charm)– central and forward rapidity regions
• High precision vertexing in the central region to identify D (cτ ~ 100-300 µm) and B (cτ ~ 500 µm) decays
ALICE(c/b)
ATLAS/CMS(b)
LHCb(b)
-2 0 2 4 61
10
100
1 year pp 14 TeV @ nominal lumin.
pT
of
Q-h
ad
ron
[G
eV
]
ηηηη of Q-hadron
ALICE(b)(c)
ALICE(c/b)
ATLAS/CMS(b)
LHCb(b)
-2 0 2 4 61
10
100
1 year pp 14 TeV @ nominal lumin.
pT
of
Q-h
ad
ron
[G
eV
]
ηηηη of Q-hadron
ALICE(b)(c)
74
Hadronic decays of D mesons
• No dedicated trigger in the central barrel extract the signal from Minimum Bias events– Large combinatorial background (benchmark study with dNch/dy = 6000 in
central Pb-Pb!)
• SELECTION STRATEGY: invariant-mass analysis of fully-reconstructed topologies originating from displaced vertices– build pairs/triplets/quadruplets of tracks with correct combination of
charge signs and large impact parameters– particle identification to tag the decay products– calculate the vertex (DCA point) of the tracks– requested a good pointing of reconstructed D momentum to the primary
vertex
D0→→→→ K-ππππ+
75
D0→ K-π+: results (I)
central Pb-Pb
~30
(for 108 evts)5%2 ⋅ 10-3
pPb
min. bias
~35
(for 107 evts* )10%5 ⋅ 10-6
Pb-Pb
Central
(dNch/dy = 6000)
pp~40
(for 109 evts)10%2 ⋅ 10-3
Significance
S/√√√√S+B
(M±1σ)
S/B
final
(M±1σ)
S/B
initial
(M±3σ)
However, with dNch/dy = 3000 in Pb-Pb, S/B larger by ×××× 4 and significance larger by ×××× 2
* 1 year at nominal luminosity:
107 central Pb-Pb events (1 month run, central events are 10% of total rate),
109 pp events (7 months run) and 108 p-Pb events (1 month run)
76
D0→ K-π+: results (II)
inner bars: stat. errors
outer bars: stat. ⊕ pt-dep. syst.not shown: 9% (Pb-Pb), 5% (pp, p-Pb)
normalization errors
Down to pt ~ 0 in pp and p-Pb (1 GeV/c in Pb-Pb)for D0 production cross-section measurement
1 year at nominal luminosity:
107 central Pb-Pb events,
109 pp events and
108 p-Pb events
77
PDFsm RFc ,,,
00 µµ
µµ
PDFsm RFc ,,,
00 µµ
µµ√√√√s = 14 TeV
Open charm in pp (D0 → Kπ) Sensitivity to NLO pQCD parameters
down to pt ~ 0 !
78
Open Beauty from single electronsSTRATEGY
• Electron Identification (TRD+TPC): reject most of the hadronsPrimary
Vertex B
e
Xd0
rec. trackB → e +X
• Impact parameter cut: high precision vertexing in ITS: reduce charm and bkg electrons
• Subtraction of the residualbackground
79
t
D
pp
t
D
AA
coll
t
D
AAdpdN
dpdN
NpR
/
/1)( =
t
e
pp
t
e
AA
coll
t
e
AAdpdN
dpdN
NpR
/
/1)( =
Charm and Beauty Energy Loss : RAA
mb = 4.8 GeV
Low pt (< 6–7 GeV/c)
Also nuclear shadowing (here EKS98)
High pt (> 6–7 GeV/c)Only parton energy loss
D0 → Kπ B → e + X
1 year at nominal luminosity
(107 central Pb-Pb events, 109 pp events)
80
Heavy-to-light ratios in ALICE
1 year at nominal luminosity
(107 central Pb-Pb events, 109 pp events)
)()()(/ t
h
AAt
D
AAthD pRpRpR =For charm:
81
Charmonia in AA collisions: from SPS and RHIC to LHC
82
J/ψ measurement in NA50 at the SPS• Aim of NA50: study the production of J/ψ in Pb-Pb collisions
• Experimental technique:
– absorb all charged particles produced in the collision except muons
– detect J/ψ by reconstructing the decays J/ψψψψ →→→→ µµµµ++++µµµµ−−−− (B.R. ≅≅≅≅ 5.9 %)
• The measured dimuon spectrum is
fitted to a source cocktail in order to
extract the J/ψ, ψ’ and Drell-Yan
contributions
Quarkonium production is usually
normalised to Drell-Yan production
(which is not influenced by strong
interactions)
83
J/ψ anomalous suppression in NA50
NA38/NA50
In peripheral Pb-Pb collisions the
(J/ψ)/DY ratio is consistent with a
normal suppression pattern (due to
the absorption of a pre-resonant c-cbar
state before the J/ψ formation)
increasing with the centrality of the
collision
In central Pb-Pb collisions
(b < 8 - 8.5 fm) a much stronger
suppression is observed:
Anomalous suppression
Anomalous suppression is
the candidate as signal for
deconfinement
New In-In data (NA60) follow the
Same pattern !!!
84
J/ψ survival probabilityS(J/ψ) = measured/expected yields (from preresonance absorption only)
SJ/ψψψψ = 0.6 SdirJ/ψψψψ + 0.4 Sχχχχc,ψψψψ’
If we take into account the fraction (~60%) of the observed J/ψψψψ which are not directly produced, but come from the decay of higher excited states (~30% from χχχχc, ~10% from ψψψψ’), we can rewrite S(J/ψψψψ) as:
Results consistent with the
full disappearance of χχχχc , ψψψψ’
Sχχχχc,ψψψψ’=0 and the survival of
the directly produced J/ψψψψSdir
J/ψψψψ =1
Included here also theexperimental resultsof NA60 (InIn at SPS) and PHENIX (AuAu at RHIC)
~ 0.6
F.Karsch, D.Kharzeev and H.Satz, hep-ph/0512239
85
Comments on J/ψ suppression at SPS and RHIC
Recent studies of the behaviour of charmonium states in a deconfinedmedium show that the charmonium ground state J/ψ survives up to 1.5 -2 Tc (corresponding to a much higher energy density than at Tc)while excited states dissolve at Tc
M Asakawa et al., Prog.Part.Nucl.Phys, 46(2001) 459 M Asakawa et al. ,PRL 92(2004) 012001S Datta et al., Int.J.Mod.Phys. A16(2001) 2215C-Y Wong, PRC 72 (2005) 034906W Alberico et al., PRD 72 (2005) 114011
The observed J/ψψψψ suppression at SPS is qualitatively very
similar to the pattern expected by deconfinement models.
And it is consistent with the hypothesis of full disappearance of thecharmonium excited states χc , ψ’ and the survival of the directly produced J/ψ.
At RHIC the J/ψψψψ suppression is compatible with that observed at SPS.However at RHIC in addition to the total suppression in the deconfinedphase it could be allowed also the regeneration of J/ψψψψ at hadronizationby recombining c-cbar.The agreement between SPS and RHIC data, would be accidental. This scenario could also occur at LHC energies.
86
Charmonia in AA collisions at LHC
• Melting of Ψ’ and χc at SPS and RHIC, and melting of J/Ψ at LHC?
• Magic cancellation between J/Ψsuppression and J/Ψ regeneration?
22 (39)% of J/Ψ (Ψ’) from open beauty meson decays.
SPS RHIC LHC
J/Ψ melting
J/Ψ regeneration
H. Satz, CERN Heavy Ion Forum, 09/06/05
????
87
Bottomonia in AA collisions at LHC
ϒ(2S) ϒ(1S)
ϒ(3S)χb(1P)χb(2P)
SPS RHIC LHC
PRD64,094015
• Regeneration is expected to be small at LHC.
• ϒ(2S) behaves as J/ψ : TD
ϒ(2S) ~ TDJ/ψ
• ϒ(1S) melts only at LHC.
• ϒ(2S)/ϒ(1S) as a function of pT.
45 (30)% of feed-down from higher resonances for ϒ(1S) (ϒ(2S))
88
• Identification of charmonia and bottonomia states through their
dilepton decay channel both in the e+e- and in the µµµµ+ µµµµ - channel
e+e- channel (TRD)
µ+ µ - channel (Muon arm)
Heavy quarkonia in ALICE
(2.5 < η < 4)
(-0.9 < η < 0.9)
• Large background from open charm & bottom
• J/ψ produced also via b decays • Secondary charmonium production from kinetic recombination and statistical hadronization• important to have good mass resolution (~ 1%) to separate the different states => perform detailed spectroscopy
89
Quarkonia→e+e- (in PbPb with ALICE)
90
Quarkonia → µµµµ++++µµµµ−−−− (in PbPb with ALICE)
• ϒ(1S) & ϒ(2S) : 0-8 GeV/c• J/Ψ high statistics: 0-20 GeV/c• Ψ’ poor significance• ϒ’’ ok, but 2-3 run will be needed.
Yields for baseline
S/(S+B)1/2S/BB[103]S[103]State
8.10.480.420.20ϒ(3S)
120.650.540.35ϒ(2S)
291.70.81.3ϒ(1S)
6.70.013003.7Ψ’
1500.20680130J/Ψ
PbPb cent, 0 fm<b<3 fm
0 1 2 3 4 5 6 7 8
En
trie
s/5
0 M
eV
/10
^6
s
210
310
410
510
610all
)uncorrb(b
)corrb(b)uncorrc(c
)corrc(c/Kπ
resonances
6 < b < 9 fm
8 8.5 9 9.5 10 10.5 11 11.51
10
210
0 1 2 3 4 5 6 7 8
En
trie
s/5
0 M
eV
/10^
6 s
210
310
410
510all
)corrb(b
)corrc(c
resonances
6 < b < 9 fm
8 8.5 9 9.5 10 10.5 11 11.51
10
210
91
Quarkonia → µµµµ++++µµµµ−−−− (in pp at 14 TeVwith ALICE)
92
Study of quarkonia suppression with ALICE
• Suppression-1 Tc =270 MeV ΤD/Tc=1.7 for J/Ψ ΤD/Tc= 4.0 for ϒ.
• Suppression-2 Tc=190 MeV ΤD/Tc=1.21 for J/Ψ ΤD/Tc= 2.9 for ϒ.
PRC72 034906(2005)hep-ph/0507084(2005)
Good sensitivity J/Ψ, ϒ(1S) & ϒ(2S)
93
Highlights on physics topics
Jet physics
94
Jet studies with Heavy Ions at RHIC
Evidence of parton energy loss at RHIC from the observed suppression of back-to-back correlations in Au-Au central collisions (and not in d-Au or p-p minbias). And also from the suppression of the RAA ratios at high pT
STAR Au+Au
√sNN = 200 GeV
1/N
trig
ger
dN
/d( ∆
φ∆
φ∆
φ∆
φ)
PRL91, 072304 (2003)
Standard jet reconstruction algorithms in nucleus-nucleus collisions at RHIC fail due to: large energy from the underlying event (125 GeV in R <0.7) limited reach up to relatively low jet energies (< 30 GeV) multi-jet production restricted to mini-jet region (< 2 GeV)
RHIC experiments use leading particles as a probe.
parton
jet
“trigger particle”
95
• It’s a final state effect (see slides: 45, 69-70)• pQCD with energy loss calculations require initial density ~30-50 times cold nuclear matter density
Eskola et al., NP A747, 511 (2005)
fmGeVq /0ˆ 2=
fmGeVq21ˆ =
fmGeVq2155ˆ −=
What did we learn from the RAA suppression at RHIC?
(no medium)
q α density
(time averaged)
96
Leading particle versus jet reconstruction
Leading Particle
Reconstructed Jet
So, ideally only the full jet reconstruction allows to measure the original
parton 4-momentum and the jet structure.
Study the properties of the QCD dense medium through modifications
of the jet structure due to the parton energy losses (jet quenching):
– Decrease of particles with high z, increase of particles with low z
– Broadening of the momentum distribution perpendicular to jet axis
Leading particle is a fragile probe
• Surface emission bias–Small sensitivity of RAA to medium properties (at RHIC, but also at LHC)
• For increasing in medium path length L, the momentum of the leading particle is less and less correlated with the original parton 4-momentum.
jet
T
T
E
pz =
Eskola et al., hep-ph/0406319
?
97
Jet rates at the LHC
• Copious production!! Several jets per central PbPb collisions for ET > 20 GeV
• Huge jet statistics for ET~100 GeV
• Multi-jet production per event extends to ~ 20 GeV
100
98
Jet energy domain
2 GeV 20 GeV 100 GeV 200 GeV
Mini-Jets 100/event 1/event 100k/month
event-by-event well
distinguishable objects
No jet reconstruction, but onlycorrelation studies (as at RHIC)Limit is given by underlying event
Reconstructed Jets
Example :100 GeV jet +underlying event
Full reconstruction of hadronic jets, even with the huge background energy from the underlying event, starts to be possible for jets with E> 50 GeV
99
ALICE detectors for jet identification
• Measurement of Jet Energy– In the present configuration ALICE measures only charged particles with
its Central Tracking Detectors(and electromagnetic energy in the PHOS)
– The proposed Large EM Calorimeter (EMCal) would provide a significant performance improvement• ET measured with reduced bias and improved resolution• Better definition of the fragmentation function: pt/ET• Larger pt reach for the study of the fragmentation of the jet recoiling from a photon and photon-photon correlations
• Excellent high pt electrons identification for the study of heavy quark jets
• Improved high ET jet trigger
• Measurement of Jet Structure is very important– Requires good momentum analysis from ~ 1 GeV/c to ~ 100 GeV/c– ALICE excels in this domain
100
Jet reconstruction in ALICE
Background energy in a cone of
size R is ~R2 (and background
fluctuations ~R).
In pp-collisions
jets: excess of transverse energy within a
typical cone of R = 1
Main limitations in heavy-ion collisions:
• Background energy (up to 2 TeV in a cone-size R=1 )
• Background energy fluctuations
They can be reduced by:
• reducing the cone size (R = 0.3-0.4)
• and with transverse momentum cut (pT = 1-2 GeV/c)
22 ϕη ∆+∆=R
background
— no pT cut
— pT >1 GeV/c
— pT >2 GeV/c
ETjet
100 GeV
150 GeV
50 GeV
30 GeV
R
E(R
)
[GeV
]
101
Background for jet structure observables: the hump-back plateau
S/B > 0.1 for ξ< 4 leading particle remnants pt>1.8 GeV
S/B ~ 10-2 for 4 < ξ< 5 particles from medium-induced gluon radiation
102
Out-of-cone fluctuations
Intrinsic jet reconstruction performance
ET=100 GeV
The limited cone-size and pt cuts (introduced to reduce background energy) lead to a low-energy tail in the spectra of reconstructed energy.This tail is enhanced if detector effects (incomplete/no calorimetry) are included
Assuming an ideal detector and applying a pt-cut of 2 GeV/c we expect, fora jet with ET=100 GeV a reconstructed cone energy of 88 GeV with gaussianfluctuations of 10%
103
Energy resolution (for ideal calorimetry)
pT > 0 GeV
1 GeV
2 GeV
ET=100 GeV
Background fluctuationsadded to signal fluctuationsfor the case pt>1 GeV/c
Cone-size 0.3 < R< 0.5 : optimal limiting resolution ∆ET/ET ~22 %
104
Photon-tagged jets
Dominant processes:
g + q → γ + q (Compton)
q + qbar→ γ + g (Annihilation)
pT > 10 GeV/c γγγγ
γγγγ energy provides independent measurement of jet energy
Drawback: low rate !!
But... especially interesting in the intermediate range (tens of GeV) where jets are not identified
Direct photons are not perturbed by the medium
Parton in-medium-modification through the fragmentation function and study of the nuclear modification factor RFF
quenched jet
non-quenched
γγγγ-jet correlation
– Eγ = Ejet
– Opposite directions
105
Summary• ALICE is well suited to measure global event properties and
identified hadron spectra on a wide momentum range (with very low pT cut-off) in Pb-Pb and pp collisions.
• Robust and efficient tracking for particles with momentum in therange 0.1 – 100 GeV/c
• Unique particle identification capabilities, for stable particles up to 50 GeV/c, for unstable up to 20 GeV/c
• The nature of the bulk and the influence of hard processes on its properties will be studied via chemical composition, collective expansion, momentum correlations and event-by-event fluctuations
• Charm and beauty production will be studied in the pT range 0-20 GeV/c and in the pseudo-rapidity ranges |η|<0.9 and 2.5< η <4.0
• High statistics of J/Ψ is expected in the muon and electronic channel
• Upsilon family will be studied for the first time in AA collisions • ALICE will reconstruct jets in heavy ion collisions
→ study the properties of the dense created medium• Furthermore, ALICE will identify also prompt and thermal photons→ characterize initial stages of collision region
Summary
106
Credits
For fruitful discussions, and for figures and slides:
F. Antinori, A. Dainese, B. Hyppolite, C. Kuhn, L. Gaudichet,M. Lopez-Noriega, G. Martinez, M. Masera, M. Nardi, F. Prino, L. Ramello, E. Scomparin and Y. Schutz.